The invention relates to a filter arrangement. The invention further relates to a transmitter, a receiver, a mobile telephone appliance with a filter arrangement, and to a wireless data transfer system, as well as to a method of manufacturing a filter arrangement.
The stormy developments in the field of mobile telephony and the continuing miniaturization of the cordless telephone appliances lead to higher requirements being imposed on the individual components. Thus a higher selectivity is necessary in the high-frequency range for protecting the receiver against the rising number of potentially interfering signals from other systems. This is achieved, for example, by means of bandpass filters which allow only a limited frequency band to pass and which suppress all frequencies above and below this band.
Among the devices which are used at the present time for this purpose are filters with ceramic electromagnetic resonators. A miniaturization of these filters, however, is limited by the electromagnetic wavelength. Much smaller designs can be achieved with so-called surface acoustic wave (SAW) filters. This is because the acoustic wavelength is smaller than the electromagnetic wavelength by 4 to 5 orders of magnitude. A disadvantage is, however, that surface acoustic wave filters often have a complicated construction and are protected by means of complicated housings. An alternative is offered by bulk acoustic wave (BAW) filters. Bulk acoustic wave filters have advantages as regards their size, power, and IC compatibility.
In practice, however, said filters do not fully filter out the frequencies lying outside the passband, but only to a certain degree. This may be insufficient for certain applications such as, for example, the signal filtering in the high-frequency part of mobile telephones or base stations.
The invention has for its object to provide a filter with a bandpass filter function which shows an improved suppression outside the passband.
This object is achieved by means of a filter arrangement which comprises a substrate on which are provided a bandpass filter and a notch filter, which filters are coupled to one another.
It is advantageous that the notch filter is connected between the input of the bandpass filter and ground or between the output of the bandpass filter and ground.
The coupling of the bandpass filter to a notch filter is capable of achieving a better suppression in the stop band of the bandpass filter at a desired frequency.
It is preferred that the bandpass filter and the notch filter are thin-film filters.
The implementation of the two filters in thin-film technology on a substrate implies that such a filter arrangement is kept within small dimensions.
It is particularly preferred that the bandpass filter comprises a filter arrangement of resonators.
It is furthermore preferred that the filter arrangement comprises bulk acoustic wave resonators, surface acoustic wave resonators, or ceramic electromagnetic resonators.
Bandpass filters composed from a filter arrangement of resonators show lower losses in the passband, because they have better quality factors Q than, for example, LC filters. Moreover, bandpass filters comprising resonators show a steeper slope at the edge of the passband than comparable LC combinations.
It is preferred in particular that a bulk acoustic wave resonator comprises a resonator unit and a reflection element which is arranged between the substrate and the resonator unit.
Such a bulk acoustic wave resonator can be manufactured by means of thin-film techniques in a simple manner and, most of all, with a small space requirement. In addition, such a bulk acoustic wave resonator is clearly more robust than other types of bulk acoustic wave resonators such as single-crystal resonators, resonators with membranes, or resonators with an air gap.
It is furthermore advantageous that the notch filter comprises a capacitor and an inductance.
Such a notch filter can be manufactured by a thin-film technique in a simple, inexpensive, and space-saving manner on the same substrate as the bandpass filter.
The invention furthermore relates to a method of manufacturing a filter arrangement, which comprises a substrate and provided thereon a bandpass filter of bulk acoustic wave resonators and a notch filter, by which method
a second electrode, a piezoelectric layer, and a first electrode are provided on a carrier layer and are structured such that at least one resonator unit, a capacitor, and an inductance are created,
a reflection element is deposited on those portions of the first electrode which belong to the resonator unit, and
a substrate is fastened on the entire assembly, and the carrier layer is removed.
The invention also relates to a transmitter, a receiver, a mobile telephone appliance, and a wireless data transfer system provided with a filter arrangement which comprises a substrate and provided thereon a bandpass filter and a notch filter, said filters being coupled to one another.